1. Field of the Invention
The invention relates to a pressure screening apparatus for screening a stock suspension and to a screen clearer.
2. Discussion of Background Information
Pressure screening apparatuses are typically used in treating paper stock suspensions, in order to process the stock suspension in a wet screening. For this purpose, the pressure screening apparatus contains a screen element which includes a number of apertures. The fibers contained in the suspension are allowed to pass through these apertures, while the undesired solid components of the suspension are prevented from doing so and are rejected and conducted out of the screening apparatus. This device may also be use for separating various grades of fiber components, i.e., separating the shorter fibers from longer fibers. Such screening apparatuses typically use round holes or slits as the grading apertures. In most cases, pressure screening apparatuses of the type under consideration here also utilize screen clearers which have clearing surfaces that are moved past the screen. This conventional design is intended to prevent clogging of the screen apertures.
A screen clearer is known from WO 98/53135 which utilizes blade elements to clear the screen. These blade elements have a hydrodynamic profile which extends over the entire length of the screen element. This design also utilizes a revolving screen. Owing to their movement relative to the surrounding suspension, the blade elements exert a pressure impulse in front of the screen and a suction impulse behind the screen. This design causes part of the suspension which was rejected at the screen or part of the suspension which has already passed the screen as accepted stock, to be sucked back. As a result of this design, the screen apertures are kept clear or are cleared. In order to effectively process suspensions having different solid contents ("consistency") in different zones of the screen element, the cross sections of the blade elements are shaped differently. However, the ability of this design to efficiently screen the stock suspension is very limited.
U.S. Pat. No. 3,586,172 discloses a pressure screening apparatus whose rotor is divided into four different zones. After a first zone, which serves only to accelerate the material, the suspension reaches a grading zone in which the rotor is equipped with elevations which are intended to fluidize the stock suspension and prevent the formation of fiber flocks ("dusters"). The subsequent grading zone is similar to the first zone and has depressions in the rotor which are intended to ensure that the flow speed is maintained and thus to prevent clogs ("plugging"). However, the clearing effect of such depressions in this design is poor.
In EP 0 289 020, it is suggested to prevent or to reduce the too rapid deckering of the residue by utilizing a rotor which accelerates the axial transport along the revolving screen. For this purpose, the rotor has a number of projections ("bulges") with oblique surfaces which produce axial flow components. Depending on the axial position of the projection in question, axial impulses of different strengths are thus produced.